1. Field of the Invention
The present invention relates generally to the field of coating fluids, such as those used in painting, in gluing and in stamp pads or marking pens. More specifically, the present invention relates to coating fluids which have reduced solvent evaporation rates when the fluids are held in open vessels, on a brush, in a porous stamp pad, an uncapped porous-tip pen or the like.
2. Description of the Prior Art
It is well-known that some liquids readily form skins on exposure to the atmosphere. This is especially common with viscous liquids such as adhesives, paints and starch-based culinary sauces. Processes which might contribute to the formation of skins include solvent loss, chemical reaction with either the oxygen or the water vapor of the atmosphere and migration of dispersed particles to the surface. It is also known that evaporation of water can be reduced by the presence of extremely thin layers of surface active materials.
Water based writing or colouring inks for use on paper commonly contain a co-solvent of low volatility which prevents the ink in the pen tip from drying out immediately. Among the liquid co-solvents commonly used are ethylene glycol, propylene glycol, diethylene glycol, glycerol and formamide. Sometimes the use of less common co-solvents such as thiodigol, propylene carbonate, hydroxethylformamide and 2-pyrrolidone might be justified. Deliquescent salts, such as sodium lactate, ammonium acetate, ammonium thiocyanate and lithium bromide can also act as low-volatility co-solvents. Furthermore, substances may be used which, although not normally liquid or deliquescent, form liquid or deliquescent mixtures with other ink components. Examples of these substances include urea, sorbitol, sulphonyldiethanol and ethylene carbonate. One drawback to inks containing such low-volatility solvent components is that they are very slow to dry on non-porous surfaces, such as films used with overhead projectors and like.
One method for formulating ink that dries quickly on non-porous surfaces but which, when used in marker pens, gives a longer "cap-off" life before the tip becomes dry is described in European Patent No. 0 359 805 (U.S. Pat. No. 5,279,652). This patent (hereinafter called the "Kaufmann, et al." patent) describes the use of certain soluble crystalline additives to make marking fluids which form a protective, but easily rupturable, layer on the surface of the fluid when contained in pens and the like. The crystalline additives are selected by a process which involves dissolving candidate materials alone in the solvent of the marker and filling the resulting solution into a pen fitted with a capillary writing tip. The pen is then left with the cap removed, and if outward growing crystals form at the tip, the additive is likely to induce such layer formation.
In the examples given in the Kaufmann, et al. patent several mechanisms may contribute to the claimed increase in cap-off life. However, an indication of what is likely to be important is provided in a description of one of its particularly preferred embodiments. In this experiment, an aqueous solution of a resin was placed in one pen with a capillary tip and a ten percent (10%) solution of crystalline solid urea was placed in a second pen. When left uncapped, the tips of both pens dried quickly, but when a mixture of the two solutions was tried in a third pen, a satisfactory cap-off life was obtained.
Marker pens with porous tips and inks which incorporate crystalline additives selected as specified in the Kaufmann, et al. patent are now on the market. In view of the observations made in the experiment described in the previous paragraph, it may be hypothesized that the inks form a protective layer in two stages. Firstly, solvent is lost from the surface of the ink in the outer pores of the tip, causing fine crystals of the additive to form in those pores. The effect is to produce a surface layer of much finer capillarity, which greatly reduces the rate of solvent exchange between the surface region and the ink below. In the second stage, as more solvent evaporates from the liquid surface, the binding resin, colourant and possibly other components are deposited and these cement the additive crystals to form a composite surface layer. Because the additive and colourant are intrinsically weak, this layer is easily ruptured during normal writing procedures.
It should be emphasized that this mechanism is hypothetical only, but it does represent a reasonable explanation of how the Kaufmann, et al. inks achieve the improvement in cap-off properties. The accuracy of such explanation does not affect the scope of the present invention.
A disadvantage of inks formulated in accordance with the Kaufmann, et al. patent is that the dried ink films do not have good gloss and they are not very transparent. Hence, the inks are not well suited for use as overhead projector pens or in similar applications. Furthermore, the Kaufmann, et al. inks do not normally form a protective layer when held in an open vessel.
Another commercially available additive which is sold by Multichem Ltd. of Hexham, England under the name "Cap Off Additive CA 210.2" and intended for use in alcohol-based marker inks, is said to be skin forming. It is a pasty solid at room temperature and substantially liquid at 40.degree. C. This material has the advantage of permitting the formulation of inks which have good gloss and transparency while increasing the cap-off life of a pen. However, the improvement in cap-off life is modest, and because the additive is a semi-solid, the use of the material can adversely affect the adhesion and cohesion of the ink deposit.
Coating fluid formulations which have significantly increased cap-off life and which have good gloss and transparency would represent a significant advance in this technology.